1. Field of the Invention
This invention relates generally to the field of circuitry used for soft starting of circuit modules. More particularly, this invention relates to a circuit which prevents sudden current spikes and voltage sags on a system bus when modules are plugged into the bus with power connected.
2. Background of the Invention
When modules are plugged into a bus connector while power is active on the bus, several undesirable things happen. Generally, there is an inrush of current to the module being plugged in which can be damaging to components on the module. Also, the supply voltage on the bus sags momentarily due to the sudden inrush of current to the module. This voltage sag can create so called "glitches" in logic circuitry causing logic errors in other circuits attached to the bus or other undesirable responses.
In the past, this problem has usually been addressed by one of three different techniques. The first technique is to place large valued inductors in series with the supply line to limit the amount of current which the module can instantaneously sink. Several problems are associated with this approach. Inductors are physically large and costly. The value required depends on the total capacitance of the module and thus is difficult to standardize for variations in types or variations in modules. Rapid variations in current requirements for the module under normal operation of the module (e.g. due to switching action taking place on the module) can be inhibited by the inductance. On removing a module using inductance as a soft start mechanism, the back EMF may cause large voltage spikes on the module causing damage to module components.
Hot powered rails with current limiting resistors have also been used to begin powering the module prior to full insertion. Unfortunately, this is a costly mechanical solution which cannot be used for all module configurations. Also, different resistance values are required depending upon the amount of capacitance on the modules, further restricting the usefulness of this technique.
So called "umbilical cord" techniques have also been used to provide initial charging of module capacitors prior to insertion. Of course, this technique has potential for human error, is expensive and complicates the module insertion procedure.
The above techniques all rely upon passive components (inductors or resistors) for accomplishing the soft start function. Accordingly they can not be used to turn off the module in a separate function. The present invention addresses the problems associated with these techniques to provide an improved active soft start mechanism which can be further utilized to provide a controlled turn on or turn off of the module under predetermined conditions.